Dinosaurs

Ceratosaurus meets BrachiosaurusThe dinosaurs were reptiles that had legs held upright beneath their bodies, rather than sprawling. They walked on either two legs or all fours. Most kinds had long tails, clawed hands and feet. Many had scaly skins, but some had a covering of feathers. Ranging from about the size of a duck to 75-tonne giants, the dinosaurs inhabited land environments on all continents—including Antarctica—during the Triassic, Jurassic and Cretaceous periods, 250–66 million years ago, together known as the Mesozoic Era. Dinosaurs were divided into two major groups: saurischians (“lizard-hipped”) and ornithischians (“bird-hipped”).

Posture

Dinosaurs stood and walked on straight legs, which were directly beneath their bodies. No other reptiles had straight, upright legs like this. Most reptiles have legs at the sides of their bodies. Modern lizards, such as the Komodo dragon, walk with a sprawling gait: they bend down at the knees.

Thecodont and CompsognathusPrehistoric reptiles called thecodonts had partly bent legs, “half-way” between an ordinary reptile and a dinosaur. Thecodonts lived before the dinosaurs. Perhaps some of them changed, or evolved, into the first dinosaurs. Crocodiles can also run in a partly-upright way. By contrast, the dinosaurs had an erect posture. The bone structure of a dinosaur’s limbs was more like that of a mammal’s. It allowed dinosaurs, such as Compsognathus, to walk and run more efficiently—even to sprint on their back legs only.